The present invention relates, in general, to the field of furnaces for heating steel plates.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
Steel plates, in particular coated plates, intended for hot forming, are typically heated in a roller hearth furnace, whereby the steel plates are placed at a certain distance upon the roller table of the furnace. The rollers are made of ceramics and continuously transport the steel plates to be heated through the furnace towards the furnace exit. The furnace is sized in length in dependence on the required cycle time, size, and material properties of the steel plates to be shaped and may range from several meters to one hundred meters and more. Although the construction of furnaces of substantial length is technically feasible, their use is however discouraged because of the substantial space demand and the problem that the loading station of the furnace is positioned at a great distance away from the press that follows the furnace so that a single worker is unable to supervise the loading station and the press at the same time. As a result, control operation becomes more complicated, and a long furnace is also impractical for energy reasons because a lengthy furnace has a relatively large surface area in relation to a small inside volume. Lengthy furnaces also require a roller table that is equally sized in length, so that the risk for malfunction rises with increase in length of the furnace. In other words, when operation of such a furnace fails, the entire heat-treatment assembly including the associated press system must be shut down.
In particular when coated steel plates are involved, a continuous advance of the steel plates is absolutely required during the heating process because otherwise at least some regions of the plate may encounter a separation of the coating. This type of damage is caused by the heat transfer between the transport rolls and the steel plates. Tests have shown that the heat transfer in a blank steel plate is promoted by the rolls. Thus there is no option other than to maintain a continuous advance of the coated steel plates on the rollers in order to evenly heat the entire steel plate.
It would therefore be desirable and advantageous to provide an improved furnace for heating steel plates to obviate prior art shortcomings and to enable a high throughput rate for large-area coated steel plates of sizes that may reach several square meters while still requiring little space.